This invention relates to new optical etalons and to methods of making and using them, as well as to devices incorporating such etalons.
Etalons are ubiquitous in optical systems, such as optical sensors, optical communication systems, etc. The basic Fabry-Perot etalon can be designed and produced to have a sharp response at resonant frequencies, which makes them suitable as optical filters, such as passband filters. They also give a variable amount of dispersion, and so have been suggested for possible use as dispersion compensators. Thus Fabry-Perot etalons are a basic building block in a number of different optical elements, i.e., in optically functional components or devices. Such devices may be active or passive and may be employed in a system (or adapted to be employed in a system) to pass or transmit a selective wavelength or band of wavelengths or periodic set of wavelength bands. Exemplary optical elements in which etalons are used include optical sensors, and filters, e.g., band pass filters, single channel filters, and other wavelength selective filter devices such as wavelength division multiplexers, and dispersion compensators and other components of optical communication systems.
Fabry-Perot etalons typically comprise precisely parallel, selectively transmissive surfaces, such as thin films, i.e., partially reflective mirrors or surfaces on opposite sides of an integral number of half waves distance or gap between them, forming the etalon""s cavity. The thin film and cavity characteristics determine the optical properties of the etalon. That is, the spectral characteristics of the etalon are generally determined by the reflectivity of the mirrors or surfaces and by the optical thickness of the cavity. Such etalons have long been produced, for example, by sputter deposition of film stacks of alternating layers of materials, i.e., a high refractive index material alternating with a low refractive index material, to form a mirror coating which is transmissive of selected wavelengths. Two such mirror coatings sandwich a sputter-deposited cavity layer between them. Sputtering or other physical deposition of the relatively thick cavity layer is time consuming and, therefore, adds substantial time and cost to the production of such etalons. The result is undesirably high cost of production for such etalons.
It has long been a recognized problem in this industry, that producing etalons having desired properties can be difficult and expensive. In addition, there are industry-recognized problems associated with producing structurally robust etalons having desired, precise optical properties. Prior known etalons have employed various designs, such as the etalons used in the interferometric optical devices of U.S. Pat. No. 6,125,220 to Copner et al. In the interleaver/de-interleaver devices of Copner et al, two glass interferometric end plates are separated by a spacer region where the etalon is formed. The spacer region is an air gap having a predetermined dimension. In adjustable Fabry-Perot devices, such as disclosed in U.S. Pat. No. 5,283,845 to Ip, tuning of the center wavelength of the spectral passband of an etalon is achieved by varying the effective cavity length (spacing) between two end plates carrying Fabry-Perot thin film reflectors. More specifically, in Ip a piezo actuator is used, extending between the two end plates. By varying the electric power applied to the piezo actuator, the axial length of the actuator can be varied, and thus the gap between the end plates varied. As alternatives to piezo-electric actuators, the tuning mechanism may include liquid crystals, temperature, pressure, and other mechanisms. It is a disadvantage that adjustable etalons as in Ip involves considerable assembly complexity and cost. Also, maintaining strict parallelism between the end plates can present additional difficulties.
The prior known optical etalons, as noted above, fail to fully meets the needs of many applications, especially for optical elements intended for optical communication systems, precision sensors, etc.
It is an object of the present invention to provide etalons addressing some of the deficiencies of the prior known technologies. It is a particular object of at least certain preferred embodiments, to provide improved etalons and methods of making same, and optical elements incorporating such etalons. Additional objects and aspects of the invention and of certain preferred embodiments of the invention will be apparent from the following disclosure and detailed description.
In accordance with a first aspect, an etalon comprises a planar bulk optic having first and second parallel, selectively transparent surfaces. The bulk optic comprises an optically transparent body and a wedge correcting coating (referred to here generally as a xe2x80x9cwedge coatingxe2x80x9d) on at least one of the two surfaces of the optically transparent body. The wedge coating, further described below, establishes high precision parallelism of the selectively transparent surfaces of the etalon. The bulk optic is a solid, optically transparent (at the wavelength or wavelengths of interest) body whose thickness, i.e., the dimension between the selectively transparent, parallel surfaces, including the wedge coating, defines the cavity spacing. In particular, the bulk optic, including the wedge coating, will typically have an optical thickness equal to an integral number of half-waves for the wavelength(s) of interest. In preferred embodiments the selectively transparent surfaces are thin film coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies.
The thickness of the wedge coating varies progressively across the etalon. That is, the thickness of the wedge coating, viewed in cross-section in at least one plane orthogonal to the parallel, selectively transparent surfaces of the etalon, has a thickness that increases (or decreases in the opposite direction) continuously, typically approximately linearly, to compensate for non-parallelism, or xe2x80x9cwedgexe2x80x9d, in the underlying body of the bulk optic. As described further below, the bulk optic can be diced from a wafer on which a wedge coating and the two thin film coatings have been deposited by magnetron sputtering, ion beam sputtering or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For an etalon intended for use as or in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm. Low wavefront error can be understood in this context to mean that the thickness of the bulk optic, i.e., the distance between the two opposite surfaces of the bulk optic, is substantially linearly variable and, hence, controllable or correctable by the wedge coating in accordance with the present disclosure. Preferably, for etalons suitable for use in optical communication elements, the wedge coating brings parallelism of the opposite surfaces of the bulk optic body from the 1 to 2 arc seconds of wedge mentioned above to less than 0.1 arc seconds, most preferably less than 0.01 arc seconds.
It is a significant advantage that the etalons disclosed here can employ a bulk optic, comprising the optically transparent body and the wedge coating, to define the cavity spacing of the etalon. Substantial cost savings and production simplification can be realized in accordance with at least certain preferred embodiments of the etalon. Further, robust and accurate etalons can be achieved using production techniques whose application will be readily understood by those skilled in the art given the benefit of this disclosure.
In accordance with a method aspect of the present disclosure, the wedge coating is deposited onto the optically transparent body of the bulk optic by physical vapor deposition, e.g., magnetron sputtering or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical 6 inch or larger substrate wafer used in the production of optical filters) not spinning during deposition. Preferably the coating is a low defect coating to allow bonding or other optical coupling. Deposition can be otherwise in accordance with known techniques, whose applicability and mariner of implementation will be within the ability of those skilled in the art given the benefit of this disclosure. The substrate is oriented at an angle to the target or otherwise arranged to receive progressively different deposition rates from one edge to the opposite edge. The target is preferably an elongate source to provide a coating with a roughly linear profile. Preferably, the target is tangential to the circumference of the substrate and is about four times as large as the substrate, e.g. for a 6 inch diameter substrate the target can be about 20 inches long. The thinnest point of the substrate is positioned closest to the target or otherwise oriented or favored to have the fastest rate of deposition. Conversely, the thickest point is positioned or oriented to have the lowest deposition rate. Thus, the sputtered material will deposit fastest and, therefore, the most heavily, i.e., the thickest, where the bulk optic was thinnest, with progressively thinner deposition toward the area where the least was wanted. Since the substrate is not spinning and is oriented or arranged as just described, the thickness of the resulting wedge coating will change progressively (hence the term xe2x80x9cwedgexe2x80x9d), with the change in the thickness of the wedge coating being opposite that of the underlying body. The net effect is that the thickness of the bulk optic is substantially uniform over all or a large portion of its area. Where the bulk optic is prepared in the form of a typical substrate wafer, Fabry-Perot thin films can then be deposited to complete the etalon (subject to any further production or packaging steps etc.) in the same or a different sputter deposition chamber. Deposition of a wedge coating and thin films on another, second surface of the bulk optic may be performed in accordance with the embodiments described here.
In accordance with another aspect, an etalon comprises a planar bulk optic having first and second selectively transparent surfaces. The bulk optic comprises an optically transparent body and a thickness-adjustment layer of substantially uniform thickness on at least one of the two surfaces of the optically transparent body. As used herein, substantially uniform thickness means the thickness of the layer across the surface of the bulk optic is approximately constant. The thickness-adjustment layer, further described below, establishes the desired thickness of the bulk optic cavity of the etalon. The bulk optic is a solid, optically transparent (at the wavelength or wavelengths of interest) body whose thickness, i.e., the dimension between the selectively transparent surfaces, including the thickness-adjustment layer coating, defines the cavity spacing. In particular, the bulk optic, including the thickness-adjustment layer, will typically have an optical thickness equal to an integral number of half-waves at same or all points for the wavelength(s) of interest. In preferred embodiments the selectively transparent surfaces are thin film coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies. As described further below, the bulk optic can be diced from a wafer on which a thickness-adjustment layer and the two thin film coatings have been deposited by magnetron sputtering, ion beam sputtering or other known deposition technique. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For an etalon intended for use as or in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm. In accordance with a further method aspect of the present disclosure, the thickness-adjustment layer is deposited by physical vapor deposition, most preferably magnetron or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical substrate wafer used in the production of optical filters) spinning during deposition in accordance with known techniques. From the disclosure above of optical elements comprising a bulk optic with a wedge coating as the cavity of the optical element, it will be apparent to those skilled in the art that embodiments of the present invention comprising a bulk optic with a thickness-adjustment layer and no wedge coating can be expected to have non-parallel surfaces carrying the optical thin films. The consequent impact on optical properties of the optical element may be addressed in accordance with suitable techniques, such as tilting of the optical element in use. Selected area of the optical element may provide optical properties meeting product specifications for an intended application. It is a significant advantage that the etalons disclosed here can employ a bulk optic, comprising the optically transparent body and the thickness-adjustment layer, to define the cavity of an etalon. Substantial cost savings and production simplification can be realized in accordance with at least certain preferred embodiments of the etalon. Further, robust and accurate etalons can be achieved using production techniques whose application will be readily understood by those skilled in the art given the benefit of this disclosure.
In accordance with another aspect, an etalon comprising a wedge coating in accordance with the above disclosure further comprises a thickness-adjustment layer. More particularly, the bulk optic comprises, in addition to the optically transparent body and the wedge coating, a thickness-adjustment layer as described above, preferably overlying the wedge coating. As noted above, the properties of an etalon will depend, in part, on the cavity spacing. Thus, in accordance with certain preferred embodiments, the thickness of the bulk optic is adjusted, typically after deposition of the wedge coating, by follow-on deposition of a thickness-adjustment layer. In accordance with a further method aspect of the present disclosure, the thickness-adjustment layer is deposited by physical vapor deposition, most preferably ion beam or magnetron sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical substrate wafer used in the production of optical filters) spinning during deposition in accordance with known techniques. In accordance with certain highly preferred embodiments, the wedge coating and then the thickness-adjustment layer are deposited onto the bulk optic (again, either alone or as part of a larger substrate) in a single continuous step. The wedge coating is deposited as described above, by deposition onto the bulk optic substrate not spinning in the vacuum chamber. Upon completion of the wedge coating, the bulk optic substrate is caused to commence spinning without interruption in deposition. Deposition is, therefore, continuous for the wedge coating and the thickness-adjustment layer. To control the optical thickness of the cavity formed by the bulk optic, the thickness can be precisely monitored during deposition of the thickness-adjustment layer. Given the benefit of this disclosure, it will be within the ability of those skilled in the art to employ suitable techniques for monitoring and controlling the thickness of the bulk optic as the thickness-adjustment layer is being deposited. Suitable optical monitoring techniques are known, including, for example, that disclosed in U.S. Pat. No. 6,110,337 to Sullivan et al., entitled Sputtering Method and Apparatus with Optical Monitoring, the entire disclosure of which is hereby incorporated by reference.
In accordance with another aspect, additional cavities can be deposited on either surface of a bulk optic etalon by suitable deposition techniques, such as, for example, ion beam sputtering, magnetron sputtering, etc. That is, additional cavities can be formed on the bulk optic etalon. More specifically, following deposition of a suitable transition layer overlying the thin film stacks on either surface of the bulk optic etalon, a cavity can be deposited by, for example, sputtering a first suitable thin film coating or stack, such as a sequence of alternating H/L/H film, followed by deposition of a cavity film, followed by deposition of a second thin film coating or stack which preferably is substantially identical to the first thin film coating. Similarly, yet one or more such thin film etalons can be deposited thereafter employing suitable odd QWOT transition layers between, in accordance with thin film principles well known to those skilled in the art. Thus, the advantage of improved optical performance characteristics of a multi-cavity device can be achieved, incorporating a bulk optic etalon of the type disclosed above together with additional thin film etalons unitary therewith by depositing such thin film etalons thereon using sputtering or other suitable deposition techniques. Any suitable materials disclosed above, and other suitable materials known to those skilled in the art, may be used for the thin films of the thin film etalons including but not limited to Ta2O5, ZrO2, TiO2, Al2O5, SiO2, and MgF. The multi-cavity deposition product, described above, is suitable for use, generally together with or in place of the single cavity embodiments of the bulk optic etalons disclosed here.
In accordance with another aspect, the etalons disclosed here may be incorporated into optical systems. Etalons can be directly optically coupled, as the term is used here, when they are optically coupled, i.e. are in the same optical path, and furthermore are in optical contact or are otherwise in physical contact with each other and/or mounted to each other (e.g. by bonding material in or out of the optical path) or mounted together in the same housing or by the same fixture. An air space may be separating the etalons or the etalons may be in direct surface-to-surface contact. Similarly, optical components in an optical system comprising the etalons disclosed here, that are directly optically coupled in accordance with the present disclosure, preferably have no intervening optical components performing substantial channel filtering or like optical operation on any passed signals. In particular, any signals passed by the etalons, or other optical components in the optical system, arrive at a second etalon, or other optical component, without any intervening wavelength filtering optical operations to add or drop passbands or like operations. One skilled in the art, given the benefit of this disclosure, will be able to design and assemble optical system comprising directly optically coupled etalons and other optical components described here.
The etalons disclosed above and the methods disclosed for their production will be recognized by those skilled in the art to represent a significant technological advance. Robust etalons can be produced meeting precise optical performance characteristics, with advantageously low production costs and good production flexibility. In preferred embodiments, the etalons have the advantageous attributes of small size, simple and potentially inexpensive construction, and good optical performance, including low loss, low polarization dependent loss and polarization mode dispersion, and low chromatic dispersion. Additional features and advantages will be understood from the following detailed description of certain preferred embodiments.